Transformation of silver nanoparticles in coagulation processes and subsequent thermal sludge treatments

Abstract

Abstract Increased use of silver nanoparticles (AgNPs) in commercial products inevitably leads to their release into the environment through sludge discharge from wastewater treatment facilities. The silver morphology and speciation in sludge plays an important role in the biotoxicity and subsequent risk assessment of AgNPs. In this study, we investigated the morphology and speciation of AgNPs after coagulation and subsequent sludge thermal treatment, treated simulated water samples that were prepared with bare and macromolecule polymers (PVP) coated AgNPs. Results from X-ray photoelectron spectroscopy and X-ray diffraction analysis demonstrated that bare AgNPs, which aggregated to large sizes before coagulation, existed as crystalline metallic silver (68.8%) and argentic chloride (31.2%) in flocs. As bare AgNPs flocs were calcinated at 600 °C, part of the metallic silver transferred to silver oxide (Ag2O). Analysis of Ag fractions showed that the percentages of dissolved silver and nano silver in bare AgNPs flocs increased drastically over 30% in acidic conditions or in the presence of calcium ions and humic acid, and further increased after calcination. In comparison, most of the PVP coated AgNPs were immobilized in flocs as Ag2O and argentic chloride. The sludge calcination did not produce any further modification in silver speciation. Comparison of the results between bare and PVP coated AgNPs demonstrated that nanoparticle aggregation before treatment is not conducive to immobilization of AgNPs in flocs, and results in health hazards and ecological risks due to the release of silver.